What do you mean by changing the sprites? Are you trying to import more sprites to use, or change the actual sprites that the character are using?
If you're just trying to add new sprites, you can click on the icon next at the top next to the icon for the scripting, and in there you'll be able to import new graphics.
If you want to change the sprites that your characters use, you can change them in the first tab of the database, by double-clicking the image of the graphic.
You can find additional sprites
here.